Handling multiple merges onto branches in Subversion or CVS is just one of those things that has to be experienced. It is inordinately easier to keep track of branches and merges in Mercurial (and probably any other distributed system) but I don't know why. Does anyone else know?

My question stems from the fact that with Mercurial you can adopt a working practice similar to that of Subversions/CVSs central repository and everything will work just fine. You can do multiple merges on the same branch and you won't need endless scraps of paper with commit numbers and tag names.

I know the latest version of Subversion has the ability to track merges to branches so you don't get quite the same degree of hassle but it was a huge and major development on their side and it still doesn't do everything the development team would like it to do.

6 Answers
6

In Subversion (and CVS), the repository is first and foremost. In git
and mercurial there is not really the concept of a repository in the
same way; here changes are the central theme.

+1

The hassle in CVS/SVN comes from the fact that these systems do not
remember the parenthood of changes. In Git and Mercurial,
not only can a commit have multiple children, it can also have multiple
parents!

That can easily observed using one of the graphical tools, gitk or hg
view. In the following example, branch #2 was forked from #1 at
commit A, and has since been merged once (at M, merged with commit B):

o---A---o---B---o---C (branch #1)
\ \
o---o---M---X---? (branch #2)

Note how A and B have two children, whereas M has two parents. These
relationships are recorded in the repository. Let's say the maintainer of
branch #2 now wants to merge the latest changes from branch #1, they can
issue a command such as:

$ git merge branch-1

and the tool will automatically know that the base is B--because it
was recorded in commit M, an ancestor of the tip of #2--and
that it has to merge whatever happened
between B and C. CVS does not record this information, nor did SVN prior to
version 1.5. In these systems, the graph
would look like:

o---A---o---B---o---C (branch #1)
\
o---o---M---X---? (branch #2)

where M is just a gigantic "squashed" commit of everything that happened between A and B,
applied on top of M. Note that after the deed is done, there is no trace
left (except potentially in human-readable comments) of where M did
originate from, nor of how many commits were collapsed together--making
history much more impenetrable.

Worse still, performing a second merge becomes a nightmare: one has to figure out
what the merge base was at the time of the first merge (and one has to know
that there has been a merge in the first place!), then
present that information to the tool so that it does not try to replay A..B on
top of M. All of this is difficult enough when working in close collaboration, but is
simply impossible in a distributed environment.

A (related) problem is that there is no way to answer the question: "does X
contain B?" where B is a
potentially important bug fix. So, why not just record that information in the commit, since
it is known at merge time!

P.-S. -- I have no experience with SVN 1.5+ merge recording abilities, but the workflow seems to be much more
contrived than in the distributed systems. If that is indeed the case, it's probably because--as mentioned
in the above comment--the focus is put on repository organization rather than on the changes themselves.

SVN 1.5+ would indeed put an SVN property on the merge commit M listing the merged commits it contains, in other words A-B. So you have the same information.
– Simon DMar 11 '10 at 11:27

Kind of. Technically, SVN's merge tracking is akin to what Git calls “cherry-picking,” with additional magic to make it easier on the user; it is semantically different from what Git, Hg and Bzr do when merging. I guess it does not make a lot of difference in practice as long as you do not care about the DAG (eagain.net/articles/git-for-computer-scientists).
– Damien DiederenMar 22 '10 at 10:35

2

Woe that there is no +100 button. Thanks.
– Charlie FlowersFeb 5 '11 at 9:07

I assume the SVN 1.5 feature you're talking about is the use of the svn:mergeinfo property?
– MatrixFrogMar 10 '11 at 7:34

@MatrixFrog: Yes, this is just what svn:mergeinfo does.
– sleskeOct 28 '11 at 9:34

Because Subversion (at least version 1.4 and below) doesn't keep track of what have been merged. For Subversion, merging is basically the same as any commit while on other version control like Git, what have been merged are remembered.

Untouched by any of the already provided answers, Hg offered superior merge capabilities because it uses more information when merging changes (hginit.com):

For example, if I change a function a
little bit, and then move it somewhere
else, Subversion doesn’t really
remember those steps, so when it comes
time to merge, it might think that a
new function just showed up out of the
blue. Whereas Mercurial will remember
those things separately: function
changed, function moved, which means
that if you also changed that function
a little bit, it is much more likely
that Mercurial will successfully merge
our changes.

Of course, remembering what was last merged (the point addressed by most of the answers provided here) is also a huge win.

Both improvements, however, are questionable since subversion 1.5+ stores additional merge information in the form of subversion properties: that information available, there's no obvious reason why subversion merge couldn't implement merge as successfully as Hg or Git. I don't know if it does, though, but it certainly sounds like subversion developers are on their way to get around this issue.

I suppose this might partially be because Subversion has the idea of a central server along with an absolute time line of revisions. Mercurial is truly distributed and has no such reference to an absolute time line. This does allow Mercurial projects to form more complicated hierarchies of branches for adding features and testing cycles by sub project however teams now need to much more actively keep on top of merges to stay current as they can't just hit update and be done with it.

In Subversion (and CVS), the repository is first and foremost. In git and mercurial there is not really the concept of a repository in the same way; here changes are the central theme.

I've not thought much about how you'd implement either but my impression (based on bitter experience and lots of reading) is that this difference is what makes merging and branching so much easier in non-repository based systems.

I only have experience with Subversion but I can tell you that the merge screen in TortoiseSVN is horribly complicated. Luckily they include a dry run button so that you can see if you are doing it right. The complication is in the configuration of what you want to merge to where. Once you get that set up for the merge the merge generally goes fine. Then you need to resolve any and all conflicts and then commit your merged in working copy to the repository.

If Mercurial can make the configuration of the merge easier then I can say that would make merging 100% easier then Subversion.